U.S. patent application Ser. No. 15/101,901 discloses a torque reaction engine (TRE), which may be used in a watercraft to achieve a fish-like motion. The resulting craft swims like a fish or marine mammal, without the myriad parts that plague other mechanical craft that attempt to swim like a fish or marine mammal.
At times it may be desirable to swim in other modes, such as like a jellyfish.